Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to the field of magnetic bearing or suspension systems. More specifically, the invention is directed towards a passive radial magnetic bearing system that includes damping elements and may also include a mechanism to provide increased stiffness at large rotor amplitudes.
2. Description of Related Art
Passive magnetic bearings are well known in the art. Many configurations of these types of bearings are possible (e.g. U.S. Pat. Nos. 5,894,181; 5,619,083; 4,072,370; 3,958,842; and 3,614,181 among others). Each of these configurations suffers from a lack of damping. Rotors supported on these types of bearings, therefore, will be poorly damped. This condition results in large vibrational amplitudes when the rotors traverse their critical speeds, increased sensitivity to imbalance forces, and decreased resistance to rotordynamic instabilities. This combination sometimes results in failure of the machines.
Many various techniques for introducing damping into passive magnetic systems have been developed (e.g. U.S. Pat. Nos. 5,910,695; 5,679,992; 5,521,448; and 5,386,166). Some of these methods have developed eddy current dampers, but these generally provide very low damping levels. An alternative method, utilized in some of the above patents, is to use a damping material, such as an elastomeric material or a woyen material, to provide the damping. Many different configurations of this approach have also been disclosed. These configurations generally rely on introducing an intermediate housing between the rotor and the machine frame. In general, the stationary portion of the passive magnetic bearing is mounted in the intermediate housing. The damping material is then positioned between the intermediate housing and the machine frame. Undesired rotor vibrational forces are transmitted from the rotor magnets to the stator magnets through the magnetic field. The transmitted vibrational forces cause movement of the stator magnets, and the intermediate housing into which the magnets are mounted. The motion is resisted by the damping material, either in shear or in compression. The resistance of the damping material to the vibrations results in frictional forces, thus dissipating the vibrational energy.
This approach has several limitations. First, the intermediate housing represents an additional component that must be manufactured and assembled, adding to system cost and complexity. Secondly, the intermediate housing has a finite, and usually substantial, mass that is added to the bearing mass. This results in a reduction in the resonant frequency of the combined bearing stator and intermediate housing, above which a loss of damping occurs. Finally, in this configuration, all of the forces transmitted through the bearing must pass through the damping element. This limits the designers"" ability to independently adjust the stiffness and damping of the bearing system to optimize rotordynamic performance.
In addition, several of these configurations rely on a single ring of magnetic material on each of the stator and rotor sections. Variations in the magnetic strength of the rotor and stator magnet materials result in variations of the magnetic forces as one ring rotates relative to the other. This results in xe2x80x9cmagnetic run-out,xe2x80x9d or a mechanical vibration of the rotor due to unbalanced magnetic forces. This sensitivity to variations in the magnetic field strength of the bearing magnets is undesirable.
It is therefore an object of the current invention to provide a passive magnetic support and damping system without the above listed drawbacks.
It is therefore a further object of the present invention to provide a passive magnetic support and damping system that is made of easily manufacturable components in a readily assemblable configuration.
It is therefore a still further object of the present invention to provide a passive magnetic support and damping system that provides increased stiffness in response to large amplitude vibrations.
It is therefore a still further object of the present invention to provide a passive magnetic support and damping system that is minimally sensitive to variations in the magnetic properties of the permanent magnet materials used.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a passive magnetic support and damping system in which the rotor portion of the damping system is comprised of a series of disks or annular rings of permanently magnetized material fixedly attached to the rotor of the machine. The stator portion is also comprised of a series of annular rings of permanently magnetized material, which are positioned concentrically with the rotor magnets. The stator and rotor magnets are formed and positioned such that a radial gap is present between said stator magnets and said rotor magnets. At least one, and preferably an even number, of the stator magnets are mounted in a damping material, which, in turn, is fixedly attached to the machine stator. This damping material may be an elastomeric material, a woven material, or any other type of material that exhibits primarily frictional losses in response to shear or compressive strains. The xe2x80x9csoft mountedxe2x80x9d stator magnet(s) provide damping to the system. The remaining stator magnets are fixedly attached to the machine stator and provide stiffness (xe2x80x9chard mountedxe2x80x9d). By varying the number, size, and magnetic strength of the stator magnets mounted in these two ways, the stiffness and damping of the bearing assembly can be varied substantially independently. Further, because only a single stator magnet is interposed between the rotor and each layer of damping material, the resonant frequency of the damping mechanism is very high (i.e. the effective mass of each damper element is minimized). This results in improved damping at higher frequencies than was available in the prior art.
An additional feature of the present invention is that the soft mounted stator magnets can be provided with a backing material that limits their displacement. When the soft mounted magnets come into contact with the backing material, they effectively become hard mounted, and contribute additional stiffness to the system. In this manner large excursions of the rotor, which cause large displacements of the soft mounted magnets, will result in increased bearing stiffness, tending to restore the rotor to the nominal position.